The present invention relates to calipers which are used to measure the edge thickness of turbine blades.
Turbine blades generally have leading and trailing edges which have a cross section contoured in the shape of a semicircle specified diameter and these edges must be accurately measured to ascertain whether the diameter of each blade edge is within tolerance limits. Because different blade styles have different nominal dimensions and tolerances for the diameters, it is also desirable to have a measuring device which is operable over a range of sizes.
In order to measure the diameter of the blade edge, it is known to employ a pin-type micrometer. However, measurement with a pin-type micrometer is made difficult because no provision is made for locating the correct measurement location on the blade edge. In measuring the diameter of the semicircle at the edge of the blade, the position of measurement should be at a distance of one-half of the semicircle diameter inward from the edge. The blade contour is usually such that the micrometer reading will be incorrect if measurement is made at a position greater or less than one-half of the semicircle diameter inward from the edge.
There has been no accurate or reliable way to locate how far into the blade contour to take the measurement of the diameter. Thus, a method is needed to overcome this source of measurement error.